Increasing energy costs and environmental concerns have emphasized the need to produce sustainable renewable fuels and chemicals. Fatty acids are composed of long alkyl chains and represent nature's “petroleum,” being a primary metabolite used by cells for both chemical and energy storage functions. These energy-rich molecules are today isolated from plant and animal oils for a diverse set of products ranging from fuels to oleochemicals.
Whereas microbial fermentation processes for producing ethanol and related alcohol biofuels are well established, biodiesel (methylesters of fatty acids) is the major long chain product produced biologically, and it is almost exclusively derived from plant oils today. However, slow cycle times for engineering oil seed metabolism and the excessive accumulation of glycerol as a byproduct are two major drawbacks of deriving biodiesel from plants. Although most bacteria do produce fatty acids as cell envelope precursors, the biosynthesis of fatty acids is tightly regulated at multiple levels and large quantities are not made. Thus, the production of fatty acids from bacteria has not yet reached the point where it is cost effective.
By introducing four distinct genetic changes into the E. coli genome, Lu et al. engineered a more efficient producer of fatty acids. Lu (2008). Their bacteria comprised (a) knocking out the endogenous fadD gene (encoding a fatty acyl-CoA synthetase) in order to block fatty acid degradation; (b) heterologous expression of a plant thioesterase to increase the abundance of shorter chain fatty acids with an eye towards improving fuel quality; (c) increasing the supply of malonyl-CoA by over-expressing ACC (acetyl-CoA carboxylase) and (d) releasing feedback inhibition caused by long-chain fatty acyl-ACPs through over-expression of an endogenous thioesterase.
Although a promising start, the authors acknowledge that considerable improvement to this strain must be made before commercial viability is attained. Furthermore, the authors obtained the fatty acids by spinning down the cells, lysing them, and extracting the fatty acids, thus the cells could not be further used for synthesis of fatty acids, further reducing the cost effectiveness of the method.
Therefore, there is a need in the art for a biological system of producing fatty acids that is more efficient and cost effective than heretofore realized. A more scalable, controllable and economic route to this important class of chemicals would be through the microbial conversion of renewable feedstocks, such as biomass-derived carbohydrates. Here we demonstrate the engineering of Escherichia coli to produce tailored fatty acids directly from simple sugars. Further, since the enzymes and pathways are well know, the methodology can be applied to other microorganisms, such as yeast or other species of bacteria.